Summary

epsilon-Sarcoglycan (Gene
SymbolSGCE) was first characterized by Ettinger
et al. (1997) and McNally
et al (1998). The gene contains 12 exons and covers 71 kb of genomic DNA on
humane chromosome 7q21.3 (mouse chromosome 6), between markers D7S644 and WI-5810 (McNally
et al [1998]).
A processed epsilon-sarcoglycan pseudogene (SGCEP) maps to chromosome 2q22.3. The mouse genome does
not contain a pseudogene. The structure of the epsilon-sarcoglycan gene is similar to that
of alpha-sarcoglycan, with nearly identical placement of intro/exon borders (McNally
et al [1998]).

The gene produces a major 1.7 kb mRNA, containing a 437 amino acid open reading frame
which encodes a typical sarcolgycan protein. Expression of the SGCE gene is
maternally imprinted. At its N-terminus it contains a hydrophobic
signal sequence, followed by a large extracellular domain, a hydrophobic transmembrane
region and a cytoplasmic domain. The extra-cellular domain contains conserved sites for
asparagine-linked N-glycosolation and four conserved Cysteine residues. The cytoplasmic
domain contains three phosphorylation consensus sites.

On Northern blot, both Ettinger
et al. (1997) and McNally
et al (1998) detected expression in all tissues examined. Levels were moderate in brain, heart, lung,
placenta and skeletal muscle and low in kidney, liver and pancreas. Using a polyclonal
antibody, Ettinger
et al. (1997) detected a 45 kDa sarcolemmal protein (in heart, kidney, liver, lung and skeletal muscle).
Expression in mouse embryos was detectable as early as tested, i.e. from E8.5 onwards (Straub
et al. [1999]). Straub
et al. (1999) characterized the DGC-complex in smooth muscle and show
that epsilon-sarcoglycan is an integral pasrt of the DGC, replacing alpha-sarcoglycan
(SGCA). A
tight interaction of beta-, delta- and epsilon-sarcoglycan is also indicated by the loss
of Sgcb- and Sgce-expression due to a Sgcd mutation in the
BIO14.6 hamster (Straub
et al. [1999]).

The epsilon-sarcoglycan gene

The human epsilon-Sarcoglycan gene (Gene
SymbolSGCE) was characterized by McNally
et al (1998). The gene was isolated by screening a genomic phage clone library with an
sarcoglycan epsilon-cDNA probe. The gene contains 12 exons and covers 71 kb of genomic
DNA (GenBank
AC069292).
One differentially spliced exon has been detected (exon 9b), which is present
rarely. McNally
et al (1998) identified a polymorphic CA-repeat in intron 3, which was polymorphic in
49% of the individuals tested.

The SGCE-gene was originally mapped to human chromosome 7q21 using the Genebridge 4
radiation hybrid panel, between markers D7S644 (8.45 cR centromeric) and WI-5810 and
D7S657 (4.2 and 7.0 cR telomeric resp., McNally
et al [1998]). On the human genome map the SGCE gene localizes to 7q21.3, it
is transcribed from telomere to centromere and flanked centromeric by the CAS1
gene and telomeric by the PEG10
gene.

A processed epsilon-sarcoglycan pseudogene was
identified by FISH, mapping to human chromosome 2q21. This localization, between
WI-3579 (3.8 cR) and WI-4650 (5.4 cR), was confirmed by radiation hybrid mapping
and from the humane genome sequence (GenBank
AC023128 from 2q22.3). The pseudogene, SGCEP, contains no introns and no intact open reading
frame. The
mouse genome does not contain a similar pseudogene. The murine Sgce gene
maps to
chromosome 6, close to the telomere, between Pon1/Pon3 (1.06 cM) and Pon 2 (2.13
cM) resp.

The structure of the epsilon-sarcoglycan gene is similar to that of alpha-sarcoglycan,
with nearly identical placement of intron/exon borders (McNally
et al [1998]). The human SGCE gene contains an alternatively spliced exon,
exon 9b. This exon shows homology with Alu-repetitive elements and is not
present in the SGCE gene of other organisms.

Legend:Exon: numbering of exons and intron/exon boundaries are according to McNally
et al (1998), except for exons 9b-11 (designated 10-12 by McNally
et al [1998]). Exon size: size of exon indicated in base pairs. Intron size:
size of intron indicated in base pairs. 5' cDNA position: first base of the exon
(according to cDNA Reference Sequence, with the first base of the Met-codon counted as position 1. Splice after: splicing occurs
in between of two coding triplets (0), after the first (1) or the second (2) base of a
triplet. Remarks: 5'UTR = 5' untranslated region, 3'UTR = 3' untranslated
region.

Early reports show maternal imprinting of the SGCE
gene. Grabowski
et al. (2003) studied the methylation pattern of CpG dinucleotides within the CpG island containing the promoter region and exon-1 of the SGCE gene.
Their data revealed that in peripheral blood leukocytes the maternal allele is methylated, while the paternal allele is
unmethylated. Most likely the maternal allele is completely methylated in brain tissue.
Surprisingly, in one affected female
that inherited the mutated allele from her mother, Grabowski
et al. (2003) found in peripheral blood only the paternal wild type allele expressed.

Muller
et al. (2002) described an apparently sporadic MDS case and two patients from an
MDS family with seemingly autosomal recessive inheritance. In both families, an SGCE mutation
was identified that was inherited from the patients' clinically unaffected fathers in an autosomal dominant fashion.
In affected individuals from one family RNA expression of only the mutated allele
could be detected while expression of exclusively the normal allele was found in unaffected mutation
carriers. An affected individual of a second family expressed both alleles.

The predicted molecular mass for SGCE, without post-translational modification and
without exon 9b, is 47 kDa; the calculated pI is 5.78. The predicted size for the mature
protein is 43.5 kDa. Endoglycosidase F treatment gave a ~1-2 kDa decrease in size of SGCE
(Ettinger
et al. [1997]),
which is consistent with the presence of one N-linked glycosylation site. Straub
et al. (1999) characterized the DGC-complex in smooth muscle and show
that epsilon-sarcoglycan is an integral part of the DGC, replacing alpha-sarcoglycan. A
tight interaction of beta-, delta- and epsilon-sarcoglycan is also indicated by the loss
of Sgcb- and Sgce-expression due to a Sgcd mutation in the
BIO14.6 hamster (Straub
et al. [1999]).

Similarity to other proteins

Human and mouse epsilon-sarcoglycan are 96% identical.

SGCE is 43% identical (63% similar) to alpha-sarcoglycan (nucleotide
level - 47% similarity within the coding region). Similarity for mouse Sgce and Sgca
extends over the whole length of the protein. A 24 amino acid stretch in the
extracellulair domain (aa168-191) and the transmembrane region (aa 283-325) are
particulary conserved (Ettinger
et al. [1997]).
The potential N-glycosylation site (Asn168) and the exact spacing of the four Cys-residues
are also conserved. The cytoplasmic domain between the two proteins is rather divergent,
suggesting that the proteins bind different partners.

epsilon-Sarcoglycan expression

Using a polyclonal antibody, Ettinger
et al. (1997) detected a 45 kDa protein on Western blots (in heart, kidney, liver, lung and skeletal
muscle). In brain tissue, two additional higher molecular mass bands were detected, 47 and
48 kDa resp. Immunohistochemistry showed a wide sarcolemmal expression of SGCE, in
agreement with the Northern and Western blot data (Ettinger
et al. [1997]).
Expression was also detected in E13.5 murine embryos, a time of development in which
SGCA-expression is absent.

Straub
et al. (1999) performed extensive studies on epsilon-sarcoglycan expression. Expression in
mouse embryos was detectable as early as tested, i.e. from E8.5 onwards. A broad
expression was detected by E12 (myoblasts, myotubes, cardiac myocytes, surrounding lung
bronchi, vascular endothelium). Neither alpha- or beta-sarcoglycan expression were
detectable at the E8.5 and E12 stages. At E15, Sgce expression was abundant in
smooth muscle of lung and bronchus, in skeletal myotubes, cardiac myocytes and a variety
of endodermal and ectodermal lineages. At E15, Sgca- and Sgcb-expression
were detectable, but restricted to skeletal and cardiac muscle.

epsilon-Sarcoglycan and disease

Using a positional cloning approach, Zimprich
et al. (2001) detected mutations in the SGCE-gene in patients with Myoclonus–dystonia syndrome (MDS; DYT11).
MDS is an autosomal dominant disorder characterized by bilateral, alcohol-sensitive myoclonic jerks involving mainly the arms and axial
muscles. Dystonia, usually torticollis and/or writer's cramp, occurs in most affected patients and may occasionally be the only symptom of the
disease. Often patients show prominent psychiatric abnormalities, including panic attacks and obsessive–compulsive
behavior. In most MDS families, the disease is linked to a locus on chromosome 7q21.
Zimprich
et al. (2001) identified 5 different heterozygous loss of function mutations in the gene for
epsilon-sarcoglycan (SGCE). Pedigree analysis showed a marked difference in penetrance depending on the parental origin of the disease allele. Of 62 clinically affected individuals (40 males and 22 females) in the combined families,
49 inherited the disease from their father and only 4 from their
mother. This is indicative of a maternal imprinting mechanism, which has
also been demonstrated in the mouse SGCE-gene.

Klein
et al. (2002) describe SGCE-changes in two families with previously
identified changes in the DRD2-
and DYT1 genes respectively. All eight affected individuals in family 1
carry changes in two genes as well as both definitely affected members in family
2. The authors are unclear whether the change in the SGCE gene or the
combination of mutations are pathogenic.